This invention relates to a vibratory transducer for measuring temperature, where the dimensions of the transducer are selected to avoid spurious modes of operation and thus provide accurate temperature readings over a wide range.
Double bar transducer elements formed in the shape of tuning forks have been proposed for a variety of uses including use as an accelerometer element (U.S. Pat. No. 3,238,789), use as a force transducer (U.S. Pat. No. 4,372,173), and use as a temperature sensor (U.S. Pat. No. 4,437,773). In the last mentioned patent, a so-called quartz thermometer utilizes the torsional mode of vibration (each tine rotates about its own longitudinal axis 180.degree. and out of phase with the other tine) to measure temperature changes. It is noted in the patent that by proper orientation of the axes of the quartz crystal used in the tuning fork--length of the tuning fork is arranged to be generally parallel to the X axis, among other things--the torsional mode frequency will change with temperature. Thus, by measuring change in the frequency of vibration, a measure of temperature change can be made.
It has been discovered that with the dimensions given in U.S. Pat. No. 4,437,773, for the tuning fork (resonator) there disclosed, a number of spurious modes of operation develop over the operational range of the device and these spurious modes result in a lowering of the mechanical Q of the device, a shift in frequency, and a possible cessation of oscillation at the desired natural resonant frequency. The cause of this anomalous behavior is that at certain frequencies, the torsional mode frequency coincides with the frequency of another vibrational mode. The coincidence of modes allows some of the electrical energy driving the desired mode to be diverted into the spurious mode resulting in an abrupt shift in frequency or cessation of oscillation, either of which renders the resonator useless as a temperature sensor.
Some of the spurious modes include (1) flexure or oscillation of the bars (tines) in phase in a direction normal to the plane of the resonator or transducer (f.sub.p), (2) flexure or oscillation of the bars 180.degree. out of phase in a direction normal to the plane of the transducer (f.sub.w) (3) oscillation in phase in the plane of the transducer (f.sub.b), and (4) oscillation out of phase in the plane (f.sub.a). In addition to the fundamental frequencies of these spurious modes there are also overtone frequencies which can likewise cause problems. These spurious modes may be excited by the pumping motion of the structure resulting from the bars flexing, by the piezoelectric effect (assuming piezoelectric material is used) if the structure's geometry is poorly chosen, and by the nonlinear elastic behavior of the transducer material. The existence of these spurious modes of operation has not been recognized in the past in torsional mode tuning forks.
Another factor to be considered for torsional mode temperature transducers is the need for configurations which will allow use of photolithography and chemical etching for fabrication. Such fabrication techniques provide cost advantages, miniaturization and tight dimensional control.